The approaches described in this section are approaches that could be pursued, but not necessarily approaches that have been previously conceived or pursued. Therefore, unless otherwise indicated, it should not be assumed that any of the approaches described in this section qualify as prior art merely by virtue of their inclusion in this section.
In wireless communications, a transmitter converts information into a signal and transmits that signal to a receiver. Often, both the transmitter and the receiver have their own phase-locked loop (PLL). A PLL is a system that allows a device to adjust a signal. For both the transmitter and the receiver, the PLL acts as a source of a clock which standardizes the timing within the waves. The PLL of the receiver is further used to adjust the received signal in certain situations. Wireless standards are used to define data included in wireless signals, modulation schemes, and actions that may be taken by PLLs of the transmitter or receiver. The PLL of the receiver adjusts received signals based on the shared wireless standard between the transmitter and the receiver.
Even assuming a perfect radio frequency transmission, the signal picked up by the receiver will not perfectly match the signal sent from the transmitter. This occurs because of unknown errors which exist between the clocks of the transmitter and the receiver. The unknown errors are defined as the carrier frequency offset (CFO) and the sampling frequency offset (SFO) and may be estimated independently.
To ensure high quality transmission, wireless standards have different ways of revealing the transmitter clock to the receiver. One method is by using known sequences in a data packet. A known sequence is a sequence of data that is defined by the specification of the wireless standard as taking place at a certain point in the transmission of the signal and containing defined data that is known to both the transmitter and the receiver. For example, in the standard for wireless transmissions at 60 GHz-IEEE 802.11ad (802.11ad), a preamble of a data packet includes two known sequences: the short training field (STF) and the channel estimation field (CEF). These two sequences use Golay fields, a particular binary phase-shifting key sequence. A binary phase-shifting key sequence uses phases with a unique pattern of binary digits which form symbols. Functionally, the two sequences contain a series of predefined symbols in a predefined order.
Recovering the clock errors and adjusting the signal accordingly is done using feedback loops. Feedback loops generally run during the entirety of a data packet, continuously sampling the received signal and making modifications to the received signal. Two feedback loops generally run on a receiver during the transmission of a data packet to estimate and correct for errors between the two clocks: one to estimate and correct for the CFO and one to estimate and correct for the SFO.
Because each data packet contains known sequences, the known sequences may be used to determine the CFO and SFO. In 802.11ad, the known sequences in the preamble of the data packet can be used to estimate the CFO and the SFO. The PLLs may estimate the CFO and SFO by determining the difference between the received preamble and the known parameters for the preamble. For example, with binary phase-shifting keys, such as the ones used in the STF and CEF of 802.11ad, the CFO may be estimated by determining the rotation of the symbols from the known STF and CEF to the received STF and CEF. Meanwhile, the SFO may be estimated by determining a delay in receipt of symbols from the known STF and CEF to the received STF and CEF. While the two offsets are mathematically related, the relationship between the two contains a different unknown phase shift for each transmission.
Running both the CFO feedback loop and the SFO feedback loop during the entirety of the data packet consumes a large amount of power. Less power can be used by lowering the gain of the system, but doing so decreases the precision with which the receiver can adjust the signal to match the signal sent by the transmitter.
What are needed are techniques for accurately estimating and correcting timing errors using techniques that utilize lower levels of power.